1. Field of the Invention
The present invention relates in general to control systems of an automotive automatic transmission, and more particularly to the control systems of a select-shock suppression type which suppresses or at least minimizes uncomfortable select-shock which would occur when a shift lever of the transmission is shifted from a neutral (N) position to a driving (D) position. More specifically, the present invention is concerned with the select-shock control systems of a type which, for reducing the select-shock, controls a hydraulic pressure applied to a corresponding engaging element of the transmission upon such N.fwdarw.D shifting.
2. Description of the Prior Art
When, under idling of an engine, an accelerator pedal is depressed just after movement of a shift lever from a neutral (N) position to a drive (D) position, racing of the engine tends to occur because of delayed rising of hydraulic pressure as compared with rising of torque and rotation speed of the engine. That is, in such case, engagement of a corresponding clutch can not keep up with the rising of the engine speed, which tends to induce an uncomfortable select-shock.
The above-mentioned undesired phenomenon will be briefly described with reference to the time charts of FIGS. 15A to 15H of the accompanying drawings. As is understood from the points "P1" of the time charts of FIGS. 15A and 15C (viz., "TH" and "PLDuty"), in the control system of the automatic transmission, a throttle opening degree (viz., depression degree of the accelerator pedal) is monitored, and a line pressure control is so made that the line pressure duty command signal "PLDuty" is instantly shifted to a higher level upon sensing a certain open degree of the throttle valve.
Thus, in the above-mentioned control system, as is understood from the points "P2" of the time chart of FIG. 15E (viz., "PL/C"), after the engine is about to race due to depression of the accelerator pedal, the hydraulic pressure for the low clutch starts to rise and instantly the clutch engaging pressure is raised by ".DELTA.P". This instant rising of the low clutch engaging hydraulic pressure however interrupts the increasing torque and increasing rotation of the engine, which causes a sudden drop of the engine torque and engine speed as is understood from the points "P3" of the time charts of FIGS. 15F and 15G (viz., "TQ" and "Ne"). As is known, such sudden drop causes the uncomfortable select-shock.